The Nipah virus (NiV) is an emerging viral pathogen, classified as a Category C pathogen of the NIAID Biodefense Research Agenda. NiV infects agricultural livestock and humans; in 1999, NiV infection among agricultural workers in Malaysia and Singapore resulted in a 40% mortality rate and loss of livestock worth over $100 million. NiV is a member of a new genus of paramyxoviruses, and little is known about NiV biology or the immune response to NiV. However, autopsy studies of NiV victims found that the virus targets lymph nodes and endothelial cells with endothelial syncytia formation being the pathologic hallmark of NiV infection. Cell fusion and syncytia formation depend on expression of the NiV F and G envelope glycoproteins. We have found that syncytia formation is regulated by glycosylation of these envelope glycoproteins, with specific N-glycans on NiV F and G playing important roles in cell fusion. Importantly, we have also found that cell fusion can be blocked by galectin-1, an endogenous lectin of the innate immune system that is expressed by endothelial cells and dendritic cells. In addition, we have made the novel observation that galectin-1 induces robust pro-inflammatory cytokine secretion by monocyte derived dendritic cells, an effect that may augment an antiviral immune response. Our driving hypothesis for this proposal is that glycan structures on the NiV F and G glycoproteins. and the innate immune lectin galectin-1 that recognizes these structures, are critical determinants in the pathogenesis of this viral disease. Our Specific Aims are: (1) Determine structural features of oligosaccharides on the F and G env glycoproteins that are critical for glycoprotein expression and syncytia formation, (2) Elucidate the mechanism by which galectin-1 regulates NiV envelope glycoprotein-mediated cell fusion, (3) Characterize the structural features involved in binding to and fusion of target cells, (4) Examine the effects of galectin-1 on soluble mediators of the innate and adaptive immune response. This work will address critical issues for rapid development of strategies to enhance innate immune responses to this emerging viral pathogen. The co-investigators have a unique combination of expertise, and propose an innovative set of studies into the glycobiology of NiV pathogenesis.